1. The mass media discourse is only a small part of the actual discourse in America. If you looked closely at the proliferation of online discussion platforms, I suspect you would see the vast majority of discussion is on non-political topics. Politics has some universal interest, however, so it is disproportionately covered by media with a universal reach.
2. We know more about what is going on in government and that leads to news coverage of the political. The easy access to information makes stories about the political easy to write so they get written. The easy access exists both because (I) politicians are elected creatures and want to reach a mass audience, and (ii) open government laws make it easier to get information about government activities than those of private entities.
3. I question whether this phenomenon is stable. Just as no TV show gets a 30 share anymore, I hypothesize that coverage of politics will become more niche just like everything else. I am not sure whether that change would be good or bad.